Integrated circuit switches used in integrated circuits can be formed from solid state structures (e.g., transistors) or micro electro mechanical systems (MEMS), formed from passive wires inside a hermetically sealed cavity. Other devices formed inside a cavity include bulk acoustic wave filters (BAW filters) or resonators (BAR); or motion detectors and accelerometers, as examples. MEMS switches are typically employed because of their almost ideal isolation, which is a critical requirement for wireless radio applications where they are used for mode switching of power amplifiers (PAs) and their low insertion loss (i.e., resistance) at frequencies of 10 GHz and higher. MEMS switches can be used in a variety of applications, primarily analog and mixed signal applications. One such example is cellular telephone chips containing a power amplifier (PA) and circuitry tuned for each broadcast mode.
For illustrative purposes, a focus on MEMS switch devices will be discussed herein, although the discussion applies to any device formed inside a cavity. Depending on the particular application and engineering criteria, MEMS structures can come in many different forms. For example, a MEMS can be realized in the form of a cantilever beam structure. In the cantilever beam structure, a cantilever arm (suspended electrode with one end fixed) is pulled toward a fixed electrode by application of an actuation voltage. The voltage required to pull the suspended electrode to the fixed electrode by electrostatic force is called pull-in voltage, which is dependent on several parameters including the length of the suspended electrode, spacing or gap between the suspended and fixed electrodes, and spring constant of the suspended electrode, which is a function of the materials and their thickness. Alternatively, the MEMS beam could be a bridge structure, where both ends are fixed.
MEMS can be manufactured in a number of ways using a number of different tools. In general, though, the methodologies and tools are used to form small structures with dimensions in the micrometer scale with switch dimensions of approximately 5 microns thick, 100 microns wide, and 200 microns long. Also, many of the methodologies, i.e., technologies, employed to manufacture MEMS have been adopted from integrated circuit (IC) technology. For example, almost all MEMS are built on wafers and are realized in thin films of materials patterned by photolithography processes on the top of the wafer. In particular, the fabrication of MEMS uses three basic building blocks: (i) deposition of thin films of material on a substrate, (ii) applying a patterned mask on top of the films by photolithography imaging, and (iii) etching the films selectively to the mask.
In MEMS cantilever type switches, the beam and other components of the switch are manufactured using a series of conventional photolithography, etching and deposition processes. In one example, a layer of sacrificial material, e.g., spin-on polymer PMGI made by Microchem, Inc. is deposited under and over the beam structure, which is vented to form a cavity. Specifically, the cavity is formed by venting the sacrificial material through vent holes. To seal the vent holes, a sealing material, e.g., oxide, is deposited within the vent holes. Alternative sacrificial cavity materials include silicon and silicon dioxide. However, it has been found that in conventional processes the sealing material deposits on the free end (moving end) of the cantilever beam, which significantly changes the stress gradient of the beam and affects the MEMS performance. This sealing process has been found to contribute the highest process variability to the build structure. Illustratively, the material on the free end of the cantilever beam can affect the pull-in voltage, the zero voltage capacitance of the beam, e.g., make the Cmin unstable, in addition to inadvertently causing actuation of the MEMS structure, upon the application of an RF signal. In still additional problems, it has been found that some cavity sealing techniques distort the beam shape and significantly affects beam shape variability. Another problem which has been identified is degraded MEMS cycling properties and MEMS beam bounce after switching.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.